1. Field of the Invention
The present invention relates to a high speed X-ray intensifying screen (hereinafter, the X-ray intensifying screen is referred to as an "intensifying screen") capable of giving good image quality.
2. Description of the Prior Art
In radiography for medical diagnosis etc., intensifying screens are used in combination with X-ray films. At present, in medical diagnosis by radiography, there is a need for a high speed radiographic system (a combination of X-ray film and an intensifying screen) for reducing the patients' dosage of radioactivity. On the other hand, there has also been a need for a radiographic system capable of providing an image quality (i.e., sharpness, granularity and contrast) adaptable for diagnosis by clinical radiography. In other words, there is a need for the development of intensifying screens which have a high speed and at the same time can provide good image qualities superior in sharpness, granularity and contrast.
Hitherto, a calcium tungstate phosphor (CaWO.sub.4) has been widely used for intensifying screens. The advantage of intensifying screens using CaWO.sub.4 phosphor is their good image quality, an attribute that derives from specific characteristics of CaWO.sub.4 phosphor. There are many other phosphors which exhibit higher luminance than that of CaWO.sub.4 phosphor under X-ray excitation, such as silver activated zinc sulfide phosphor (ZnS:Ag). However, under the radiographing conditions usually used for medical diagnosis the intensifying screens using phosphors other than CaWO.sub.4 phosphor are inferior in image quality, especially in granularity, but they are also generally lower in contrast. Compared with the other two properties, granularity and contrast, the image sharpness is not greatly dependent upon the kind of phosphor used. For example, the sharpness can be improved by reducing the grain size of the phosphor used or reducing the thickness of the fluorescent layer. However, the granularity and contrast depend largely on characteristics specific to the phosphor and the degree of granularity and contrast of the intensifying screen are fundamentally determined by the specific phosphor used.
In his Japanese Patent Application Nos. 75,267/1976; 75,268/1976; 120,895/1976; 120,896/1976; and 123,288/1976, the applicant previously proposed intensifying screens using complex halide phosphors represented by the following composition formula EQU (Me.sub.1-f,Mg.sub.f)F.sub.2.aMe'X.sub.2.bKX'.cMe"SO.sub.4 :dEu.sup.2+,eTb.sup.3+
wherein Me, Me' and Me" each represents at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium; X and X' each represents at least one of chlorine and bromine; and a, b, c, d, e and f are numbers defined by one of the following five combinations;
1. 0.80.ltoreq.a.sub.1 .ltoreq.1.50, 0.10.ltoreq.b.sub.1 .ltoreq.1.50, c.sub.1 .dbd.0, 0.001.ltoreq.d.sub.1 .ltoreq.0.20, e.sub.1 .dbd.0 and f.sub.1 .dbd.0;
2. 0.30.ltoreq.a.sub.2 .ltoreq.1.50, 0.10.ltoreq.b.sub.2 .ltoreq.2.00, 0.01.ltoreq.c.sub.2 .ltoreq.1.00, 0.001.ltoreq.d.sub.2 .ltoreq.0.20, e.sub.2 .dbd.0 and f.sub.2 .dbd.0;
3. a.sub.3 .dbd.1, b.sub.3 .dbd.0, c.sub.3 .dbd.0, 0.01.ltoreq.d.sub.3 .ltoreq.0.10, 0&lt;e.sub.3 .ltoreq.0.05 and f.sub.3 .dbd.0;
4. a.sub.4 .dbd.1, b.sub.4 .dbd.0, c.sub.4 .dbd.0, 0.001.ltoreq.d.sub.4 .ltoreq.0.20, e.sub.4 .dbd.0 and 0&lt;f.sub.4 .ltoreq.1; and
5. a.sub.5 .dbd.1, 0&lt;b.sub.5 .ltoreq.1.5, c.sub.5 .dbd.0, 0.001.ltoreq.d.sub.5 .ltoreq.0.20, e.sub.5 .dbd.0 and 0&lt;f.sub.5 .ltoreq.1
prepared by activating a complex host material comprising, as the essential constituting components, an alkaline earth metal fluoride and an alkaline earth metal halide (hereafter, the host material of phosphor is referred to as a "complex halide") with divalent europium (Eu.sup.2+) or divalent europium and trivalent terbium (Tb.sup.3+), that is, complex halide phosphors including the following five kinds of phosphors represented by the following composition formulae:
1. MeF.sub.2.a.sub.1 Me'X.sub.2.b.sub.1 KX':d.sub.1 Eu.sup.2+
wherein Me and Me' each represents at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium; X and X' each represents chlorine and bromine; and a.sub.1, b.sub.1 and d.sub.1 are numbers meeting the condition of 0.80.ltoreq.a.sub.1 .ltoreq.1.50, 0.10.ltoreq.b.sub.1 .ltoreq.1.50 and 0.001.ltoreq.d.sub.1 .ltoreq.0.20. (Hereinafter called "phosphor-I")
2. MeF.sub.2.a.sub.2 Me'X.sub.2.b.sub.2 KX'.c.sub.2 Me"SO.sub.4 :d.sub.2 Eu.sup.2+
wherein Me, Me' and Me" each represents at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium; X and X' each represents at least one of chlorine and bromine; and a.sub.2, b.sub.2, c.sub.2 and d.sub.2 are numbers meeting the condition of 0.30.ltoreq.a.sub.2 .ltoreq.1.50, 0.10.ltoreq.b.sub.2 .ltoreq.2.00, 0.01.ltoreq.c.sub.2 .ltoreq.1.00 and 0.001.ltoreq.d.sub.2 .ltoreq.0.20. (Hereinafter called "phosphor-II")
3. MeF.sub.2.Me'X.sub.2 :d.sub.3 Eu.sup.2+,e.sub.3 Tb.sup.3+
wherein Me and Me' each represents at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium; X represents at least one of chlorine and bromine; d.sub.3 and e.sub.3 are numbers meeting the condition of 0.01.ltoreq.d.sub.3 .ltoreq.0.10 and 0&lt;e.sub.3 .ltoreq.0.05. (Hereinafter called "phosphor-III")
4. (Me.sub.1-f.sbsb.4,Mg.sub.f.sbsb.4)F.sub.2.Me'X.sub.2 :d.sub.4 Eu.sup.2+
wherein Me and Me' each represents at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium; X represents at least one of chlorine and bromine; and d.sub.4 and f.sub.4 are the numbers meeting the condition of 0.001.ltoreq.d.sub.4 .ltoreq.0.20 and 0&lt;f.sub.4 .ltoreq.1. (Hereinafter called "phosphor-IV")
5. (Me.sub.1-f.sbsb.5,Mg.sub.f.sbsb.5)F.sub.2.Me'X.sub.2.b.sub.5 KX':d.sub.5 Eu.sup.2+
wherein Me and Me' each represents at least one of the alkaline earth metals of the group consisting of barium, strontium and calcium; X and X' each represents at least one of chlorine and bromine; and b.sub.5, d.sub.5 and f.sub.5 are numbers meeting the condition of 0&lt;b.sub.5 .ltoreq.1.5, 0.001.ltoreq.d.sub.5 .ltoreq.0.20 and 0&lt;f.sub.5 .ltoreq.1. (Hereinafter called "phosphor-V")
The intensifying screens using the aforesaid complex halide phosphors have a speed or sensitivity which is a few times higher than that of the intensifying screens using the CaWO.sub.4 phosphor and a sharpness of the same level as the intensifying screens using the CaWO.sub.4 phosphor. However, the intensifying screens using these complex halide phosphors are inferior in granularity and contrast and so improvement in these respects has been desired.